Created singleshot capability and related logic

Pablo Ariñorequested to merge
pabarino/akonadi:feature/add_singleshot_capability into master

Added support for SingleShot agent capability

Agents that declare the SingleShot capability in their .desktop file will automatically shut down after completing their work and emitting the finished() signal. This is useful for agents designed to perform one-off tasks and then terminate, reducing background resource usage. Related to MR

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